Fault sensing instrumentation

ABSTRACT

An electrochemical sensor system includes an electrode assembly that produces a DC signal as a function of a parameter of interest sensed by the electrode system. An AC signal is applied via an electrode to the conductor fluid in which the electrode system is disposed and an AC signal detector is connected to the DC circuitry. A threshold circuit responsive to the output of the AC signal detector indicates fault in the electrochemical sensor system whenever the AC signal detector has an output of predetermined magnitude.

i United States Patent 51 May 9, 1972 Blackmer [54] FAULT SENSINGINSTRUMENTATION [72] Inventor: David E. Blackmer, Harvard, Mass.

[73] Assignee: Instrumentation Laboratory, Inc., Lexington, Mass.

[22] Filed: Apr. 7,1970

[21] Appl. No.: 27,197

[52] US. Cl. 204/195, 204/1 T, 204/195 B, 324/29, 324/30 R [51] int. Cl..B01k 3/00 [58] Field ofSearch ..204/l T, 195 B, 195 F, 195 P;

[56] References Cited UN lTED STATES PATENTS 3,345,273 10/1967 Brown..204/1 T Primary Examiner-John H. Mack Assistant Examiner-W. I. SolomonAttorney-Willis M. Ertman [5 7] ABSTRACT 7 Claims, 3 Drawing FiguresPATENTEDMAY 91912 SHEET 1 OF 3 FIG I IOO PATENTEDHAY 9 I972 SHEET 3 BF 3FIG 3 IOO FAULT SENSING INSTRUMENTATION SUMMARY OF INVENTION Thisinvention relates to instrumentation useful for the mea surement ofsmall DC voltages or currents and more particularly to systems fordetecting faults in electrochemical sensor mechanisms used in suchinstrumentation.

A significant change in the resistance of a sensor used inelectrochemical instrumentation, for example for measuring partialpressures of hydrogen, oxygen, or carbon dioxide, frequently indicatesimproper operation or failure of a part of the sensor. In some cases,failure may be heralded by an increase in the electrical resistance ofthe sensor while in other cases failure may be indicated by a decreasein the electrical resistance of the sensor. Where failures are of such anature that the DC quantity sensed (voltage or current) increases beyondusual values, it is possible to use a biased threshold failure detectionsystem. Such detection systems however prevent use of that portion ofthe dynamic range of the sensor above the threshold. Also, such a systemis not useful when the parameter sensed becomes smaller rather thanlarger, particularly where small quantities are measured in normaloperation.

Accordingly it is an object of this invention to provide novel andimproved fault detection systems for use with electrochemicalinstrumentation that measures small DC voltages or currents produced byelectrochemical sensors.

Another object of the invention is to provide novel and improved faultdetection system for use with an electrochemical instrumentation thatsenses a plurality of different parameters.

Still another object of the invention is to provide novel and improvedinstrumentation for detecting faults in electrochemical sensors of thetype that employ permeable membranes.

A further object of this invention is to provide novel and improvedfault detection systems for use in instrumentation of the type formeasuring partial pressure of oxygen and pH of blood.

In accordance with the invention there is provided an electrochemicalsensor system that includes an electrode system for disposition in aconductive fluid that is arranged to produce a DC signal as a functionof a parameter of interest sensed by the electrode system, and DCcircuitry responsive to the DC signal from the electrode system forproducing an output indicative of the parameter of interest sensed bythe electrode system. The system for detecting a sensor fault comprisesmeans for providing an electrical connection to the conductive fluid,mans to apply an AC signal to the electrical connection, an AC signaldetector connected to the DC circuitry, and a threshold circuitresponsive to the output of the AC signal detector for providing anoutput signal indicative of a fault in the electrochemical sensor systemwhen the AC signal detector has an output that differs by apredetermined amount from a normal value.

In a particular embodiment, the invention is incorporated in acomprehensive instrument arranged to sense pH, p and pCO values of bloodsamples. The instrumentation includes separate sensor electrodeassemblies for the three parameters of interest, each assembly beingconnected to separate DC circuitry for operating an output device todisplay an output indicative of the value of the particular sensedparameter. An AC signal source is provided which has a low voltageoutput and a higher voltage output. The low voltage output is applied tothe conductive fluids in which the sensor electrodes are immersed andthe higher voltage output is applied to AC signal detector circuitry. Inthe detector system used with the p0 and pCO sensors, a fault isindicated whenever the sensor resistance becomes so small that the ACsignal transmitted through the DC circuitry exceeds a predeterminedvalue; while in the pH system, the detector indicates a fault wheneverthe source resistance becomes so large that the AC signal is attenuatedbelow a predetermined value. In these arrangements the AC signal derivedfrom the sensor through the DC circuitry is compared with an AC signalfrom a common source so that the effect of variation of the magnitude ofthe output from the AC source is eliminated.

It is preferred to employ a phase sensitive detector which permitsselection of a more precise threshold value, discrimination againstinterfering AC signals from other sources; and discrimination betweentypes of signals that may be applied to the AC detector circuitry.

The invention provides sensitive and reliable detection of conductionfaults in electrochemical sensor systems. Other objects, features andadvantages of the invention will be seen as the following description ofa particular embodiment of the invention progresses, in conjunction withthe drawings, in which:

FIG. 1 is a block diagram of an electrochemical sensor systemconstructed in accordance with the invention; HO. 2 is a schematicdiagram of portions of the p0, and pCO; sensor systems of the embodimentshown in FIG. 1; and

FIG. 3 is a schematic diagram of portions of the pH sensor system of theembodiment shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT The system shown in FIG. 1 includesstructure 10 having inlet and outlet passage 12 and 14 and a chamber 16in which a fluid sample may be positioned. Two electrode systems arepositioned for insertion into contact with the fluid in chamber 16, a p0electrode system 20 and a pCO electrode system 22. A water jacket 24surrounds the cuvette assembly to maintain the chamber 16 at a constanttemperature environment. Each electrode assembly 20, 22, includes amembrane 30, 32 which is permeable to the molecules of interest,membrane 30 being permeable to oxygen molecules and. membrane 32 beingpermeable to carbon dioxide molecules. Each membrane separates thesample chamber 16 from an associated electrode compartment 34, 36 whichcontains a saline solution. Cathode electrode 38 projects through thewall of compartment 34 and its tip is exposed at the end of compartment34. Anode electrode 40 surrounds compartment 34. Ag/AgCl electrode 42extends into chamber 36 and a reference electrode 44 is disposed in asurrounding chamber which contains an electrolyte liquid. When theelectrodes are of the proper materials and a potential of the properpolarity is-applied to the electrodes, a current flows between theelectrodes which is proportional to the partial pressure of theparameter of interest. The parameter difi'uses through the membranes 30,32 and causes current flow between the electrodes which is amplified byamplifiers 50, 52, respectively, and an indication is developed bymeters 54, 56, respectively.

With respect to the pHsensor, the solution to be measured is in chamber60 and immersed in that container is another container 62 made of glassand containing an ionized solution 64 into which an electrode 66 isimmersed. When the pH of the solution in chamber 60 is different fromthat of solution 64, a potential difference appears between the oppositesides of the pH sensitive portion of glass 62. A reference electrode 68is immersed in the solution in chamber 60 and the resulting DC potentialdifference between electrodes 66 and 68 is proportional to thedifference between the pI-Is of solutions in chambers 60 and 62 plus aconstant. This DC potential difference is amplified by amplifier 70 andindicated by output meter 72.

An AC signal from oscillator is applied to these electrode systems, thesignal being applied to electrode 82 that is in contact with chamber 16and to electrode 84 in chamber 60 (or alternatively to referenceelectrode 68). Should either membrane 30 or 32 leak, the liquid inchamber 16 can affect the accuracy of the reading. As each membrane isan excellent insulator when it is of proper integrity, only a minute ACcurrent can flow from electrode 82 through either membrane 30 or 32 andelectrodes 38 and 40 to the measuring circuit. However, if the liquid inchamber 16 contains ions, when a leak develops, sufficient AC currentcan flow through the leak to produce a significant AC voltage acrossresistor 85. This resulting signal is amplified by an amplifier 86,rectified by rectifier 88 and applied to the threshold circuit 90 sothat when the AC signal ex ceeds a fixed magnitude, threshold circuit 90produces an output indicating a defect in the electrode system. In thepH electrode system, if the impedance between electrodes 66 and 68becomes higher than the input impedance of amplifier 70, as for examplewhen either electrode 66 or 68 is removed from container 60, the ACvoltage at the amplifier is much reduced. This AC voltage is rectifiedby rectifier 92 and compared in a threshold circuit 94. When thatvoltage drops below a A predetermined value, indicator 96 is turned onshowing that the pH electrode 62 has been removed from or is making poorcontact with the solution in the container 60.

A schematic diagram of a preferred embodiment of the invention is shownin FIGS. 2 and 3. In that embodiment, the oscillator 80 produces a 170Hertz signal on two output lines, a 100-millivolt signal on line 100 anda 24 volt signal on line 102. The one hundred millivolt signal isapplied to electrode 82 in the cuvette chamber 16 and to the referenceelectrode 68 via coupling capacitor 104.

In the oxygen electrode 20, the leakage to the cathode area is critical.The cathode 38 is connected through an impedance transforming amplifier1 to an operation amplifier stage 112 so that the input is maintained atvirtualground. AC gain of the amplifier is controlled by the feedbackpath that includes resistor 114 and capacitor 116. DC gain is controlledby the feedback resistor 118. The output of amplifier 112 is coupledthrough a further operational amplifier stage 120 to a servo controlcircuit 124 for operating a display of the value sensed by oxygenelectrode assembly 20.

The anode electrode 40 is connected to a network of resistors 130, 132and capacitor 134 which applies to a 0.6 volt polarizing signal to theelectrode system 20.

Also connected to the output of amplifier 112 is capacitor 150 andresistor 152. The junction between capacitor 150 and resistor 152 isconnected to an amplifier stage that includes transistors 154 and 156. Aphase sensitive demodulation circuit is connected to the output of theamplifier stage and includes coupling capacitor 158 which is connectedto the collector 160 of transistor 162. The emitter 164 of transistor162 isgrounded and the base electrode is connected through resistor 166to the 24 volt square wave signal from oscillator 80. The demodulatedoutput signal from the demodulation circuit is coupled by resistor 170to the output amplifier which includes transistors 172, 174. A lamp 176,connected in the collector circuit of transistor l74provides anindication of the condition of the membrane 30 of the oxygen electrode20.

This circuit and the component values as indicated energizes lamp 176when the current flowing to capacitor 150 exceeds a value of about twonanoamperes, corresponding to a membrane resistance of less than 50megohms.

The carbon dioxide electrode 22 has apl-I electrode 42 connected toimpedance transforming amplifier 200 and its reference electrode 44connected to impedance transforming amplifier 202. Also connected to thereference electrode is a circuit that includes capacitor 204 andresistor 206 which has an effective AC impedance of about 100 kilohms.Resistor 208 establishes a relationship to ground for the electrodesystem 22. The impedance transforming amplifiers are connected to adifferential amplifier arrangement that includes operational amplifiercircuits 210 and 212. The output of the differential amplifier isapplied to a derivative circuit that includes operational amplifier 214followed by an antilog circuit 216 whose output is applied to a servocontrol circuit 218.

An AC signal is from the output of amplifier 202 is coupled by capacitor220 connected to resistor 222 to two stage AC amplifier 224. The outputof the AC amplifier is applied to a demodulation circuit 226. A 24 voltsignal from oscillator 80 is applied over line 102 through resistor 228to the base electrode of a transistor in the demodulation circuit 226.As in the case of the circuitry coupled to the oxygen electrode 20,these circuit values are selected so that the capacitive component ofthe membrane conductance is rejected. The output signal is appliedthrough an output amplifier 230 which has a lamp 232 connected in thecollector circuit of its. output transistor. In this circuit, a voltagein excess of 20 millivolts is applied to the capacitor 220(corresponding to a membrane leakage impedance of less than 400,000ohms) will energize lamp 232.

In the pH electrode system as shown in FIG. 3, the pH electrode 62 has atypical impedance of in the order of 100 megohms and is connected to animpedance transforming amplifier 250 through a shielded conductor 252.The shield of conductor 252 is connected through a guard circuit that includes capacitor 254 and resistors 256, 258 to the output of amplifier250 to reduce the effect of the capacitance of the cable 252.

The reference electrode 68 is connected to impedance transformingamplifier260. Also connected to the input of amplifier 260 is a networkthat includes resistor 262 and capacitor 104. The 100 millivolt signalfrom line 100 is applied to reference electrode 68 via capacitor 104 andalso passes through amplifier 260 for application to operationalamplifier 270 that has a gain of two. The output from amplifier 250 isconnected to operational amplifier 272 which is connected in adifferential amplifier arrangement with amplifier 270 and its output isapplied to a servo control circuit 274 that has a servo clamp input 276.

The output of operation amplifier 270 and the output of impedancetransforming amplifier 250 are connected together through a resistancenetwork of resistors 282 and 284. The junction between resistors 282 and284 is connected through capacitor 286 to the input of an AC amplifierthat includes transistors 288 and 290. The output of the AC amplifier iscoupled through resistor 292 and capacitor 294 for modification bydemodulation circuitry including transistor 296. The 24 volt AC signalfrom oscillator is applied through capacitor 298 and resistor 300 to thebase of transistor 296. The output of the demodulator is coupled byresistor 302 to a twostage output amplifier that includes transistor 304and 306 and the output of that amplifier is coupled by resistor 308 tothe servo clamp input 276. The indicated component values in thisembodiment are selected to apply a plus l2 volt clamping voltage to theservo circuit 274 when the AC signal output of amplifier 250 is lessthan about 68 percent of the check signal applied to reference electrode68 on line 100. The sizes of resistors 282 or 284 determine the locationof the decision point at which the servo circuit 274 is clamped.

Thus this embodiment incorporates fault detection for three differenttypes of electrochemical sensors, and produces a check on the properoperation of each sensor as a function of a particular characteristic ofthat sensor. A superimposed AC signal is employed to sense the status ofthe sensor. Through the use of a demodulation signal derived from thesame source as the AC signal applied to the electrode systems, greatersensitivity is achieved. Further, in the preferred embodiment describedabove the phase angle of the demodulator is controlled so that theresistive component of the electrode membrane conductance is detectedand the capacitive component is rejected. The system of the inventionthus provides particularly sensitive monitoring of electrochemicalsensors. Additional details of a blood analysis system in which thisapparatus is incorporated are set out in copending application, Ser. No.27,200, entitled Fluid Analyzing Apparatus" filed April 7, 1970 in thename of Spergel et al. and assigned to the same assignee as thisapplication.

While a particular embodiment of the invention has been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiment or to details thereof anddepartures may be made therefrom within the spirit and scope of theinvention.

What is claimed is:

1. In an electrochemical sensor system having an electrode system fordisposition in a conductive fluid and arranged to produce a DC signal asa function of a parameter of interest sensed by said electrode system,and DC circuitry responsive to said DC signal for producing an outputindicative of the parameter of interest being sensed by said electrodesystem,

a system for detecting a sensor defect comprising:

means for providing an electrical connection to said conductive fluid,

means to apply an AC signal to said electrical connection,

a phase sensitive AC signal detector connected to said DC circuitry,

and a threshold circuit responsive to the output of said AC signaldetector for providing an output signal indicative of fault in saidelectrochemical sensor system when said AC signal detector has an outputof a predetermined value.

2. The system as claimed in claim 1 and further including means forapplying a reference signal from said AC signal source to said phasesensitive detector.

3. The system as claimed in claim 1 wherein said phase sensitivedetector includes means for discriminating against the capacitivecomponent of the AC signal passed through said DC circuitry from saidelectrode system.

4. The system as claimed in claim 1 and further including meansresponsive to said threshold circuit output signal for producing acontrol function.

5. An electrochemical sensor system having an electrode system fordisposition in a conductive fluid and arranged to produce a DC signal asa function of a parameter of interest sensed by said electrode system,said electrode system including an active electrode and a referenceelectrode, DC circuitry responsive to said DC signal for producing anoutput indicative of the parameter of interest being sensed by saidelectrode system, an impedance network connected to said referenceelectrode, said impedance network determining a threshold value, meansfor providing an electrical connection to said conductive fluid, meansto apply an AC signal to said electrical connection, an AC signaldetector connected to said DC circuitry, and a threshold circuitresponsive to the output of said AC signal detector for providing anoutput signal indicative of fault in said electrochemical sensor systemwhen said AC signal detector has an output of predetermined valuerelative to said threshold value.

6. An electrochemical sensor system having an electrode system fordisposition in a conductive fluid and arranged to produce a DC signal asa function of a parameter of interest sensed by said electrode system,said electrode system including an anode electrode and a cathodeelectrode, DC circuitry responsive to said DC signal for producing anoutput indicative of the parameter of interest being sensed by saidelectrode system, said DC circuitry including an operational amplifierof the trans-resistance type, an AC gain network connected in thefeedback loop of said operational amplifier for establishing a thresholdlevel, means for providing an electrical connection to said conductivefluid, means to apply an AC signal to said electrical connection, an ACsignal detector connected to said DC circuitry, and a threshold circuitresponsive to the output of said AC signal detector for providing anoutput signal indicative of fault in said electrochemical sensor systemwhen said AC signal detector has an output of predetermined valuerelative to said threshold level.

7. An electrochemical sensor system having an electrode system fordisposition in a conductive fluid and arranged to produce a DC signal asa function of a parameter of interest sensed by said electrode system,said electrode system including a pH electrode and a referenceelectrode, DC circuitry including a differential amplifier responsive tosaid DC signal for producing an output indicative of the parameter ofinterest being sensed by said electrode system, an AC signal detectorconnected to said DC circuitry, means for applying an AC signal to saidreference electrode, a difference network connected between the outputsof said pH and reference electrodes for applying a difference signal tosaid AC signal detector, the component values in said differencecircuitry determining a threshold value and a threshold circuitresponsive to the output of said AC signal detector for providing anoutput signal indicative of fault in said electro-chemical sensor systemwhen said AC signal detector has an output of predetermined valuerelative to said threshold value.

2. The system as claimed in claim 1 and further including means forapplying a reference signal from said AC signal source to said phasesensitive detector.
 3. The system as claimed in claim 1 wherein saidphase sensitive detector includes means for discriminating against thecapacitive component of the AC signal passed through said DC circuitryfrom said electrode system.
 4. The system as claimed in claim 1 andfurther including means responsive to said threshold circuit outputsignal for producing a control function.
 5. An electrochemical sensorsystem having an electrode system for disposition in a conductive fluidand arranged to produce a DC signal as a function of a parameter ofinterest sensed by said electrode system, said electrode systemincluding an active electrode and a reference electrode, DC circuitryresponsive to said DC signal for producing an output indicative of theparameter of interest being sensed by said electrode system, animpedance network connected to said reference electrode, said impedancenetwork determining a threshold value, means for providing an electricalconnection to said conductive fluid, means to apply an AC signal to saidelectrical connection, an AC signal detector connected to said DCcircuitry, and a thReshold circuit responsive to the output of said ACsignal detector for providing an output signal indicative of fault insaid electrochemical sensor system when said AC signal detector has anoutput of predetermined value relative to said threshold value.
 6. Anelectrochemical sensor system having an electrode system for dispositionin a conductive fluid and arranged to produce a DC signal as a functionof a parameter of interest sensed by said electrode system, saidelectrode system including an anode electrode and a cathode electrode,DC circuitry responsive to said DC signal for producing an outputindicative of the parameter of interest being sensed by said electrodesystem, said DC circuitry including an operational amplifier of thetrans-resistance type, an AC gain network connected in the feedback loopof said operational amplifier for establishing a threshold level, meansfor providing an electrical connection to said conductive fluid, meansto apply an AC signal to said electrical connection, an AC signaldetector connected to said DC circuitry, and a threshold circuitresponsive to the output of said AC signal detector for providing anoutput signal indicative of fault in said electrochemical sensor systemwhen said AC signal detector has an output of predetermined valuerelative to said threshold level.
 7. An electrochemical sensor systemhaving an electrode system for disposition in a conductive fluid andarranged to produce a DC signal as a function of a parameter of interestsensed by said electrode system, said electrode system including a pHelectrode and a reference electrode, DC circuitry including adifferential amplifier responsive to said DC signal for producing anoutput indicative of the parameter of interest being sensed by saidelectrode system, an AC signal detector connected to said DC circuitry,means for applying an AC signal to said reference electrode, adifference network connected between the outputs of said pH andreference electrodes for applying a difference signal to said AC signaldetector, the component values in said difference circuitry determininga threshold value and a threshold circuit responsive to the output ofsaid AC signal detector for providing an output signal indicative offault in said electro-chemical sensor system when said AC signaldetector has an output of predetermined value relative to said thresholdvalue.